1. Field of the Invention
This invention relates to a method of, and an apparatus for, producing ceramic moldings such as ceramic honeycomb structures.
2. Description of the Related Art
A ceramic honeycomb structure 8 having a large number of cells 88 partitioned by partitions 81, as shown in FIG. 12 of the accompanying drawings, has been used as a catalyst support of an exhaust gas purification device of an automobile. Ceramic moldings having a ceramic honeycomb structure are generally produced through extrusion molding.
A production apparatus 9 for ceramic moldings according to the prior art includes a screw type extruder 91 and a mold 93 fitted to the distal end of the extruder 91 through a resistance pipe 92 as shown in FIG. 13. A ceramic material 80, that is pressure-fed from the extruder 91 into the resistance pipe 92 with the revolution of an extrusion screw 911, is extruded from the mold 93 as a ceramic molding having a desired shape. The extrusion screw 911 is so disposed as to continue into a de-airing chamber 912 that is evacuated to a vacuum. The ceramic material 80 fed into the deairing chamber 912 by a push-in screw 913 is supplied to the extrusion screw 911 by a pair of right and left push-in rollers 914.
When a ceramic molding has a complicated shape such as a honeycomb structure, the resulting shape is greatly affected by variance of the slit hole shape of the mold 93, variance of the viscosity (variance of temperature) of the ceramic material 80, and so forth. The viscosity of the ceramic material 80, in particular, changes depending on season, time zone, etc, invites variance of the flow rate of the material, and exerts great influences on the shape of the ceramic molding.
To cope with such problems, Japanese Unexamined Patent Publication (Kokai) No. 9-277234 (prior art technology 1), for example, proposes to furnish a guide ring, that constitutes a mold, with a temperature controller for regulating the temperature of the ceramic material immediately before molding. This temperature controller can finely regulate a molding rate of the portion that serves as an outer skin of the honeycomb structure, and is believed to be capable of preventing molding defects.
Japanese Unexamined Patent Publication (Koukai) No. 53-21209 proposes to set the outer peripheral temperature of the ceramic material to a temperature by at least 10xc2x0 C. higher than its center temperature between the extrusion screw and the mold (prior art technology 2). This technology is believed capable of molding continuously an excellent ceramic honeycomb structure.
Regarding the honeycomb structure, however, a demand has been increasing in recent years for reducing the thickness of partitions so as to improve its cell density. To reduce the thickness of the partitions, the extrusion resistance of the mold 93 becomes much greater than in the conventional apparatuses. The increase of the extrusion resistance makes the influences of variance of the viscosity of the ceramic material, and so forth, on the molding shape greater than ever.
Therefore, even if variance of the viscosity (temperature) of the ceramic material has not been the problem in the molding operation of the honeycomb structure having a conventional size, this variance exerts great influence on the molding shape when a honeycomb structure having a reduced thickness is produced. Consequently, it is difficult in this case to produce a honeycomb structure having an excellent shape at a high production yield.
When those ceramic moldings which are difficult to mold, such as a honeycomb body having a reduced thickness, are molded, even the methods of the prior art technologies 1 and 2 cannot smoothly produce the ceramic moldings having an excellent shape because their shape correcting effect is too small.
An explanation will be given more concretely. In the method of the prior art technology 1, the portion for regulating the temperature of the ceramic material is only the guide ring portion of the mold. Therefore, this method can regulate the temperature of only a limited outermost surface portion. If the shape defect of the molding is very serious, the method cannot easily correct the shape.
In the method of the prior art technology 2, the temperature control is limited to a certain specific condition, and only heating is available. It is therefore extremely difficult by this method to correct the shape of moldings in such a fashion as to follow accurately slight changes of the temperature, the moisture content, the grain size, etc, of the ceramic material.
In view of these problems of the prior art, it is therefore an object of the present invention to provide a method of, and an apparatus for, producing ceramic moldings that can smoothly execute a molding operation even for those moldings, the molding of which is believed to be difficult, while suppressing molding defects.
In a method of producing a ceramic molding having a desired shape by using a production apparatus including a screw type extruder and a mold connected to the distal end of the extruder through a resistance pipe, and by extruding a ceramic material, that is pressure-fed from the extruder into the resistance pipe, from the mold, the present invention provides a method of producing a ceramic molding characterized in that the ceramic material pressure-fed from the extruder into the resistance pipe is heated or cooled from round the periphery of the resistance pipe in order to control the shape of the ceramic molding extruded from the mold.
In other words, the ceramic material positioned inside the resistance pipe is positively heated or cooled in order to improve the shape of the ceramic molding.
Various means can employed as the heating or cooling means, such as means for circulating a heat medium round the resistance pipe, means for disposing a heating device such a heater round the resistance pipe, means for disposing a cooling device such as a cooler round the periphery of the resistance pipe, and so forth. The heating or cooling means can be controlled either manually or automatically in accordance with the shape of the ceramic molding.
The ceramic molding may be a honeycomb structure having a large number of cells. The ceramic honeycomb structure is relatively difficult to form. However, when the excellent molding method described above is employed, smooth molding can be easily carried out while suppressing molding defects. Particularly when the honeycomb structure has a cell density of 300 to 1,500 cells/inch2 or a cell partition thickness of 0.035 to 0.125 mm, or when the cell density is 300 to 1,500 cells/inch2 and the cell partition thickness is 0.035 to 0.125 mm, the function and effect of the production method described above becomes remarkable. The cell shape may be various shapes such as square, triangular, hexagonal, and so forth.
In the present invention, heating or cooling of the ceramic material is preferably carried out by circulating a heat medium, that is heated or cooled, round the resistance pipe, and by changing at least one of the circulation flow rate of the heat medium and its temperature. In this case, heating or cooling can be controlled easily.
In the present invention, the heat medium is preferably caused to circulate along a spiral flow passage round the resistance pipe. In this case, heat transfer can be achieved efficiently from the heat medium to the ceramic material inside the resistance pipe.
In the present invention, the heat medium is preferably circulated from the side of the mold towards the extruder on the periphery of the resistance pipe. When the heat medium is circulated spirally, for example, the heat medium is gradually moved from the mold side towards the extruder side while being wound round the resistance pipe. In this way, the temperature control of the ceramic material can be conducted more uniformly.
It is further preferred in the present invention to measure the temperature of the ceramic mold, that is extruded from the mold, at its outer peripheral portion and at its center, and to change the temperature of the heat medium so that the temperature difference between the outer peripheral portion and the center remains constant. According to this arrangement, the temperature of the heat medium can be controlled precisely in accordance with a change of the shape of the ceramic molding.
Next, in an apparatus for producing a ceramic molding having a desired shape, including a screw type extruder and a mold connected to the distal end of the extruder through a resistance pipe, by extruding a ceramic material, that is pressure-fed from the extruder into the resistance pipe, from the mold, the present invention provides an apparatus for producing a ceramic molding characterized in that material temperature regulation means for heating or cooling the ceramic material pressure-fed from the extruder into the resistance pipe is disposed round the resistance pipe.
It is most noteworthy in the production apparatus of the present invention that the material temperature regulation means is disposed round the resistance pipe.
Means having various structures may be used as the material temperature regulation means, as will be explained later.
Using the material temperature regulation means described above, the production apparatus of the present invention can positively heat or cool the ceramic material positioned inside the resistance pipe in accordance with the shape of the ceramic molding to be extruded. In order words, the excellent production method described above can be easily executed by using this apparatus. In other words, even when the ceramic molding is the one whose molding is believed to be difficult, the production apparatus can execute smoothly the molding operation by suppressing molding defects.
In the present invention, the material temperature regulation means preferably comprises a fluid circulation passage disposed round the resistance pipe, for circulating the heat medium and a medium supply circuit connected to the fluid circulation passage, whereby the medium supply circuit preferably includes a temperature controller for heating or cooling the heat medium and a flow rate regulator for regulating the flow rate of the heat medium. In this case, heating/cooling control can be carried out precisely when at least one of the temperature of the heat medium and its flow rate is changed.
In the present invention, the fluid circulation passage preferably comprises one chamber or space encompassing the periphery of the resistance pipe, and fins for limiting the travelling of the heat medium are preferably disposed in the space so that the heat medium can be circulated spirally. In this case, the structure of the fluid circulation passage can be simplified, and the fins can easily circulate the heat medium spirally. As the heat medium is circulated spirally, the heat transfer can be made efficient from the heat medium into the resistance pipe.
In the present invention, the fluid circulation passage preferably comprises a plurality of partitioned spaces disposed round the resistance pipe, and the heat medium can be allowed to circulate into each space. In this case, an excellent shape correction effect can be acquired even when the ceramic molding has an odd-shape such as an elliptic sectional shape, for example. In other words, local heating/cooling can be made by applying heating or cooling under the difference condition for each partitioned space of the fluid circulation passage. Consequently, the shape correction effect can be obtained in accordance with the odd-shape.
In the present invention, the fluid circulation passage can be disposed inside a tubular body that is wound spirally round the resistance pipe. In this case, a spiral heat medium passage can be easily formed as the tubular body is wound round the resistance pipe.
In the present invention, the fluid circulation passage is preferably disposed from the mold side towards the extruder side round the periphery of the resistance pipe. This arrangement makes it possible to execute temperature control of the ceramic molding more uniformly.
In the present invention, the production apparatus preferably includes a molding thermometer for measuring the temperature of the outer peripheral portion of the ceramic molding extruded from the mold and the temperature at its center, and a heat medium temperature instruction device for calculating the difference of the actual measurement temperature between the outer peripheral portion and the center from the measurement values obtained from the molding thermometer, comparing the difference with a set temperature difference set in advance, and calculating a target temperature of the heat medium, wherein the temperature controller is controlled on the basis of the target temperature outputted from the heat medium temperature instruction device. In this case, the temperature controller can be controlled accurately in accordance with the change of the shape of the ceramic molding because the molding thermometer and the heat medium temperature instruction device are used.
The present invention may be more fully understood from the description of preferred embodiments of the invention set forth below, together with the accompanying drawings.